The film's heroines are a pair of sisters, Caroline and Edith Grant, whose cottage is visited by not one but two infiltrators, first a dying paratrooper and then a living specimen, convincingly posing as a British army officer. Caroline's alertness catches the imposter's slip - he mispronounces 'Jarvis Cross' as 'Yarvis Cross' - and though he manages to overpower her, he is eventually apprehended by the Local Defence Volunteers (Home Guard) and the invasion is foiled. The two sisters win through by adhering to the rules: they keep maps under lock and key and don't give out information to strangers, and when the impostor does escape, he can't get far because their car and bicycle are both immobilised. It's exactly this sort of foresight, the film shows, that will be Britain's best defence: "the front line is in every home nowadays". Some officials in the War Office considered the film too frightening, but its premise - the enemy in the heart of a docile English village - was later reused in the most famous 'invasion' film of the war, Ealing's Went the Day Well? (1944).